Such a device is known from EP0956121. This patent document describes how in a device a gas and a liquid are contacted by moving a liquid and a plate, which is provided with gas-throughput openings provided with projecting channels, into opposite directions, for example, by flowing the liquid along the surface of the plate, which is inclined for this purpose. This flow creates a venturi effect resulting in a negative pressure in the downstream directed openings of the gas-throughput openings. The bubbles of gas that in this manner are sucked through these openings into the liquid have substantially no overpressure relative to the surrounding liquid. As a result they do not expand, and do not quickly escape towards the surface of the liquid flow but are carried along as elongated gas bubbles in the liquid flow over a relatively long path. In addition to the resulting intense gas-liquid contact, this method offers the advantage that the gas can be fed to the gas-throughput openings at a relatively slight overpressure.
However, the mass transfer and overall performance of a device described above was found not to be satisfactory, in particular when treating “difficult” liquids, such as foaming liquids and/or liquids having a relatively high viscosity or cohesion or a relatively high concentration of solids.
From research it emerged that in the downstream parts of the relatively long path along which the liquid flows over plates provided with the gas-throughput openings, a considerably lower mass transfer occurred than in the upstream parts thereof.
It appeared that when treating said liquids the conditions, desirable for a good venturi action, such as uniform thickness, velocity and impact of the film-shaped flow of liquid, became disturbed already after a relatively short path over the plate. It was found from applications in the field and further research, that the required uniform thickness of the film-shaped flow of liquid was disrupted when, after a relatively short path over an inclined plate, this flow contracts and becomes thicker to the longitudinal axis of the plate causing the liquid film on both sides of said thickening to become thinner. Where thin spots in the liquid film are formed, this film may disrupt entirely (‘break’) with the result that a part of the gas that flows through the gas-throughput passages can flow through the apparatus without contacting the liquid. This problem occurs to a greater extent in case of liquids with higher viscosity and/or cohesion.
Another cause for the lower mass transfer in the downstream part was found to reside in the considerably lower gas flow rate through the gas-throughput openings, caused by a lower venturi effect in this location and, as a consequence, by a lower negative pressure in these openings which are downstream directed. It was found that the flow becomes saturated with gas bubbles already after a relatively short stretch of the plate. Since such a gas-saturated flow does not have the impact which is desirable for a sufficient venturi effect, the gas throughput at the more downstream portion of the plate is considerably lower than at the more upstream portion. The velocity of the liquid flow, desirable for a proper venturi effect, is adversely affected during its flow over the plate resulting in a loss of kinetic energy which loss is caused by the venturi action. This loss of kinetic energy, in combination with a reduction of the impact and mass of this flow, results in that the venturi action, and as a consequence the gas flow through the gas-throughput openings, is considerably lower at the more downstream part of a relatively long inclined plate than at the more upstream portion. It was found that these effects reduce the gas throughput and the mass transfer of the device. In order to enable the treatment of the above ‘difficult’ liquids and widen the field of application this problem has to be solved.
A further effect of the above-described decrease of the venturi action at the more downstream part of the plate and the saturation of the liquid film with gas bubbles, occurring there, is that the gas supplied at low pressure is not, or only partly, sucked into the flow and as a consequence forms a gas layer underneath this flow which lifts it locally. This lifting of the liquid flow also results in that a part of the gas, that flows through the gas-throughput openings, passes the device without contacting the liquid. This effect also occurs to a larger extent at liquids with a high content of solids and relatively viscous liquids with a strong cohesion.
In U.S. Pat. No. 3,830,706 a method is described to redistribute a film of liquid again over the width of a plate by passing it over inclined plates provided with apertures, and encountering baffles. The baffles have been arranged such that they, together with the plates form a slit. Through the slits the liquid films may be redistributed over the width of the plates. Although the uniformity of the film may be restored by this method, the baffles cause the flow to lose velocity and turbulence which is desired for a good venturi action and optimal gas-liquid contact. Therefore the use of a slit-shaped gap between a baffle and the plate did not provide a good venturi action and hence failed to improve the mass transfer.
As described in EP 0956121 elongated gas bubbles are being formed over a relatively long path which bubbles are entrained in the liquid flow. Therefore, a substantial part of the gas escapes from the liquid only when this arrives in the liquid-collection reservoir which is located at the downstream end of the inclined plates on the underside of the devices according to this prior art. Depending on the composition of the liquid, this leads to the formation of a foam layer which is built up from this reservoir so that gas-throughput openings are covered by foam. This foaming is also disadvantageous because the gas stream supplied to the apparatus is contacted with the surface of the foam which is accumulating in the liquid-collection reservoir and with the liquid which is present therein, so that the gas is moistened. This results in that the solids that may be present in the liquid, will deposit and accumulate on the downward facing side of the plates provided with gas-throughput openings, so that these openings can clog. It is clear that this will have a negative influence on the mass transfer.